Process for metal wire drawing and a tool for actuating the process

ABSTRACT

In an improved process for drawing metal wires and a tool for actuating the process, a metal wire is coated with a thin surface layer of lubricant and subsequently drawn, gradually in a series of single drawing operations. In a final of the drawing operations, the wire is forced to pass through a hole which exhibits an angle of convergence comprised between 30 degrees and about 85 degrees. As the wire passes through the hole, the layer of lubricant is removed and the drawn wire exiting from the hole is clean and lubricant-free.

The present application is the national stage under 35 U.S.C. 371 ofPCT/IT99/00211, filed Jul. 9, 1999.

TECHNICAL FIELD

The invention relates to a drawing process where the metal wire iscoated with a thin surface layer of material to improve drawing, thewire being passed through several different drawing holes.

BACKGROUND ART

A process of the above-described type is known, in which the wire iscoated with a lubricant which serves both to reduce friction during theforced passage through the draw-plate and to obtain a wire with a smoothsurface and a constant section. Prior-art drawplates exhibit a reductiontract, where the metal wire is subjected to plastic deformation. Thisreduction tract has an angle of convergence which is normally comprisedbetween 12 and 14°, but in any case never exceeds 20-25°. EP-A-0 537 618discloses a process for metal wire drawing in which a metal wire,previously coated by a layer of material for aiding the subsequentdrawing operation, is drawn by means of a series of drawing operations.The wire passes through a series of draw-plates in order to reach itsfinal section size. The draw-plates have holes each exhibiting areduction tract which produce a plastic deformation of the wire. Theangle of convergence of the reduction tracts is less than 25°.

This is due to the fact that drawplates wit holes having an angle ofconvergence above this produce irregular drawing results thanks toexcessive traction forces on the wire, which can be exacerbated to thepoint of breakage in further drawing operations. In any case, suchlevels of traction lead to rapid wear on the wire. Prior-art drawingprocesses of the above type generally comprise a further phase, afterthe last of the drawing operations, in which the wire is treated toremove the surface layer of lubricant. This leads to several drawbacks.Firstly, the process is made complicated by the need to predisposesuitable equipment for removing the lubricant from the wire after theprocess; secondly, working times are considerably affected, withconsequent increases in manufacturing costs.

OBJECT AND SUMMARY OF THE INVENTION

The main aim of the present invention is to eliminate theabove-described drawbacks by providing an improved drawing process inwhich a surface layer of the wire can be removed simply andeconomically.

An advantage of the invention is that after the final drawing stage asubsequent phase is not necessarily required in order to remove acoating from the wire. A further advantage of the present invention isthat a high-quality metal wire is obtained, having a smooth externalsurface and straight constant transversal section.

A further aim of the present invention is to provide a tool which simplyand economically removes a surface layer of coating from the metal wirebeing drawn.

BRIEF DESCRIPTION OF THE DRAWINGS

Further characteristics and advantages of the present invention willbetter emerge from the detailed description that follows of a preferredbut non-exclusive embodiment of the invention, illustrated purely by wayof a non-limiting example in the accompanying figure of the drawing, inwhich:

FIG. 1 shows a schematic and partial longitudinal section of adraw-plate according to the invention during use.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) OF THE INVENTION

With reference to FIG. 1, 1 denotes in its entirety a tool for drawingmetal wires, which comprises a draw-plate with a hole 2 having alongitudinal axis x—x, an inlet 3 and an outlet 4 for a metal wire 5.Arrows 6 indicate the wire advancement direction as it passes throughthe tool 1. The tool 1 is in fact the last draw-plate of a seriesthereof through which the wire passes in order to reach its finalsection size. Using Roman numerals I to V, five planes orthogonal to theaxis x—x are indicated, arranged one after another in the advancementdirection 6 of the wire 5 and distanced one from another so that theyidentify 4 distinct but coaxial zones of the drawplate tool 1.

The first zone, comprised between planes I—I and II—II, is cone-shapedin order for the wire to be introduced; for this reason the angle ofconvergence can be quite large. This first cone I—I performs no plasticdeformation on the wire 5, merely introducing it into the body of thetool 1.

The second zone, comprised between II—II and III—III, comprises aconverging reduction cone, with an angle α of convergence which in theembodiment illustrated is about 32-33°. The wire 5 is subjected toplastic deformation in this zone. This is in fact the most importantpart of the draw-plate tool 1, as it is here that the wire section isreduced.

The angle of convergence of the first zone (introduction cone) canadvantageously be equal to the angle of convergence of the second zone(reduction cone). The too cones could be made into one continuous cone,with no change in the angle of convergence, which would make the tool 1easier to manufacture.

The third zone, comprised between planes III—III and IV—IV, is aconstant-section cylinder, having a same diameter as the wire diameterD2 on completion of drawing, i.e. as it exits the output 4. The fourthzone, comprised between plates IV—IV and V—V, is an outlet cone, with arelatively large angle of divergence which does not interactcontactingly with the drawn metal wire 5.

In other embodiments, not illustrated, of the invention, the angle α ofconvergence of the second reduction zone can be different from the oneshown in FIG. 1; in the further embodiments, the angle α of convergenceis preferably comprised between 30 and 85 degrees and is in any casemore than about 30°. Also possible would be angles α of convergenceconsiderably greater than 85°, indeed, it would be possible to useangles of up to 120° and above.

The tool 1 can be used to actuate a metal wire drawing process as in thepresent invention. The process involves coating the wire to be drawnwith a thin layer 7 of material which aids the subsequent drawing. Theabove-mentioned material might be, for example, a lubricant reducingfriction during drawing, giving a drawn wire with a smooth surface andconstant section.

The process reduces the section of the wire 5 by a series of gradualdrawing operations. In the first operations, but excluding the finalone, the drawing holes have a relatively small angle of convergence, forexample between 12° and 14°, and in any case less than 25°. In the finaloperation, the tool 1 as described herein is used, with an angle ofconvergence as shown between planes II—II and III—III, exhibiting arelatively high angle α of convergence, greater than 30° and preferablybetween 30° and 85°. Angles α above 85° can be used.

The metal wire 5 at the inlet 3 to the final draw-plate, i.e. the tool1, has a diameter D1, while at the outlet the diameter is D2, smallerthan D1. Furthermore the metal wire 5 at the inlet 3 has a layer ofsurface coating 7, relatively thin, made, for example of a lubricatingmaterial. Experiments have shown that when the metal wire 5 passesthrough the hole 2 of the draw-plate 1, the layer of surface coating 7does not pass trough, being stopped at the start of the reductionpassage from II—II to III—III and being removed to form a waste scarfwhich rolls back towards the inlet 3 of the hole 2, whence it can beeasily removed.

At the outlet 4 the metal wire 5 exhibits a diameter D2 which is smallerthan D1 at the inlet 3, and further exhibits a smooth external surfacecleaned of the layer 7 of material it presented at the inlet 3.

The tool 1 therefore has a double function; firstly it reduces thesection of the wire, through a drawing action; then it removes the layerof lubricant coating the wire, through a sort of scouring of thelubricant.

The choice of the shape of the tool 1 (especially the choice of theangle α of convergence at the inlet 3 and the outlet 4 of the reductioncone) depends on various factors. In particular it is necessary to finda proper balance among the following three parameters: the percentage ofreduction of the wire section; the degree of wire cleanliness required(which increases proportionately as the angle α increases) and theworking life of the tool (which increases as angle α is reduced),

What is claimed is:
 1. A process for drawing a metal wire (5),comprising the steps of: coating a layer (7) of material on a metal wire(5) to aid a subsequent drawing thereof, drawing the metal wire (5)through a through hole in each of a series of drawplates before drawingthrough a final drawplate, each said through hole of the series ofdrawplates having a reduction tract and an angle α of convergence lessthan 25°; a final through hole of the final drawplate having a reductiontract with an angle (α) of convergence greater than 30°; wherein drawingof the metal wire (5) through the reduction tract of the series ofdrawplates and the final drawplate produces plastic deformation of themetal wire (5).
 2. The process according to claim 1, wherein the finalthrough hole has a reduction tract having an angle (α) of convergencebetween 30° and 85°.
 3. The process according to claim 1, wherein thefinal through hole has a converging zone before a reduction tract in thefinal drawplate which has an angle (α) of convergence equal to that ofthe reduction tract in the final drawplate, the converging zone does notcontribute to plastic deformation of the wire (5) and is joinedcontinuously to the reduction tract in the final drawplate so as to forma single converging tract therewith.
 4. An apparatus for metal wiredrawing, comprising a series of drawplates with holes through which awire passes in order to each final section size thereof, said holes eachhaving a reduction tract which produce a plastic deformation of thewire, said holes other than that of a last drawplate of the series eachexhibiting an angle of convergence which is less than 25°, wherein thelast drawplate of the series has at least one drawing hole (2) having areduction tract exhibiting an angle (α) of convergence greater than 30°.5. The apparatus of claim 4, wherein the angle (α) of convergence of thelast draw plate is between 30° and 85°.
 6. The apparatus of claim 4,wherein the angle (α) of convergence of the last drawplate is greaterthan 85°.